PURPOSE: To constitute a hollow for stabilizing oscillation frequency and a waveguide-MIC converter simultaneously by loading a dielectric substrate that forms a microwave integrated circuit MIC constituting an oscillating circuit in a square waveguide one end of which is short-circuited.

CONSTITUTION: A hole 21 is provided at a position bisecting the length L of wider face of a square waveguide one end of which is short-circuited, and a dielectric substrate 5 is inserted to the hole. The first strip conductor 23 forming the first probe part 22 that changes propagation mode electromagnetic field of the waveguide and propagation mode electromagnetic field of a microstrip, the second strip condcutor 25 that forms the second probe part 24 and the third strip conductor 26 for obtaining specified feedback are provided on one face of the dielectric substrate 5. The first, second, and third terminals 1, 2, 3 of a transistor 1 are connected to the first, second and third strip conductors 23, 25, 26 respectively, and other face of the dielectric substrate 5 is covered with a grounding conductor 27.